4+2] Cyclohexane Ring Formation by a Tandem of a Free Radical Alkylation of a Non-Activated δ-Carbon Atom and Intramolecular Carbanion Cyclo­alkylation

Abstract

A[4+2] cyclohexane ring formation was achieved by the combination of free radical and ionic reaction sequences. Free radical alkylation of the remote non-activated δ-carbon atom involves addition of 8-carbon radicals, generated by 1,5-hydrogen transfer in alkoxyl radical intermediates, to the radicophilic olefins, while the polar sequence involves the enolate anions as intermediates which undergo a cycloalkylation reaction. The cyclohexane rings were constructed using diverse acyclic compounds 15 and 18 as well as cyclic alkyl arenesulfenates (e.g., 5, 24, 27) as the precursors of alkoxyl radicals (four-carbon atom fragment) and methyl vinyl ketone or other activated olefins as two-carbon atom fragments. Annulation of the cyclohexane ring was applied for the synthesis of a variety of cyclic systems including monocyclic (17 and 20), fused-rings (e.g. 23, 26, 29) and spirocyclic systems (7).